A glass substrate used in a conventional flat panel display has advantages such as high heat resistance, transparency, and blocking property, but may be broken if the glass substrate is dropped, and has no flexibility and is heavy even with a thin thickness. Accordingly, various researches into a flexible display that is capable of replacing the glass substrate have been in progress. Polyimide widely used in the flexible display has been widely used not only for various electronic device fields such as an interlayer insulating film for a semiconductor, a buffer coat, a substrate for a flexible printed circuit, a liquid crystal alignment film, an electrical insulating material, a substrate for a flexible display, and the like, but also for high-tech with high heat-resistant materials such as automotive materials, aerospace materials, and the like, due to excellent properties such as mechanical properties, heat resistance, chemical resistance, electrical insulation, and the like.
In general, colorless transparent polyimide is usable as materials for manufacturing transparent and highly heat resistant coating films or films in various electronic device fields such as an electrode insulating film, a transparent protective film, a hard coating layer of a touch panel in a liquid crystal display field, a hard coating layer on a transparent flexible substrate, a transparent film material, and the like. As the related art documents preparing the polyimide, Korean Patent Laid-Open Publication No. 10-2008-055544 (Jun. 19, 2008) discloses a colorless transparent polyimide resin including 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (HBDA) as aromatic dianhydride and including either of diamine and fluorine-containing diamine having flexible groups as aromatic diamine or compounds of two or more thereof, and Korean Patent Laid-Open Publication No. 10-2013-0061404 (Jun. 11, 2013) discloses a diamine compound in which aromatic diamine is linked to an aryl group through linking groups consisting of an ester group or an amide group, and preparation of polyamic acid and polyimide prepared by using the diamine compound. Further, Korean Patent Laid-Open Publication No. 10-2015-0078618 (Jul. 8, 2015) discloses a diamine compound in which two aromatic diamines are linked to alkylene through ring linking groups, and preparation of polyamic acid and polyimide prepared by using the same.
The polyimide prepared by using the diamine compound disclosed as the related art has a high birefringence due to an aromatic ring structure with high density, and accordingly, there is limitation in replacing a glass substrate for LCD with a polyimide substrate. As a method for realizing a low birefringence of polyimide, there is a method of preparing polyamic acid by using a fluorine-based compound or an aliphatic compound, and then preparing polyimide by thermal cyclization of the polyamic acid. However, the method of using the fluorine-based compound and the aliphatic compound may improve birefringence, but remarkably deteriorates heat resistance, such that the method is required to be improved.
Therefore, the present applicants found that highly heat-resistant polyimide is capable of being prepared by using a novel diamine monomer compound obtained by introducing aromatic ring substituents including electron-accepting heteroatoms and halogen atoms into electron-donating biphenyl diamine to minimize intramolecular charge mobility, thereby maximizing transparency of the polyimide prepared by using the same and implementing a high refractive index and a low birefringence at the same time, and completed the present invention.